1. Field of the Invention
The present invention relates to a lighting device which light-emitting diodes (LED) are mounted in rows, and a manufacturing method of a substrate for lighting device.
2. Description of the Related Art
In recent years, a prevention of global warming is called out, and many countermeasures are taken in the world. Among them, specifically, each company advance the development of a technology to generate energy such as solar battery and wind-power generation, to store energy such as fuel battery and to save energy such as inverter using energy with high efficiency as the technology preventing carbon dioxide evolution.
As one of the technology to save energy, LED is pushed into the spotlight because LED can be drove with quite electrical power saving compared to filament lamp and fluorescent light. Thus, LED is used as backlight of larger-screen LCD TV, lighting for home use and car headlights.
However, this LED has a problem in terms of temperature characteristic. For example, this problem is shown in FIG. 5 of JP-A-2001-203395.
According to the explanation of the above art, as shown in FIG. 16, LED has characteristics that an amount of light does not increase, but decrease even if driving current is increased once surface temperature of LED chip (operating range of temperature) become above from about 80 to 90 degrees C.
A bottom curve shows a change of current versus surface temperature on Al substrate, a curve plotted by the middle triangles shows a change of current versus surface temperature on PCB substrate, and a curve having a peak upward shows a change of current versus amount of light of LED. Because the curve of the amount of light has a peak, it seems difficult to increase the amount of light of LED without reducing the temperature of LED as much as possible and making the amount of light increase against driving current. In case that a printed substrate as a mounting substrate of LED is employed, a surface temperature of printed substrate rises 230 degrees C. at a time, and the amount of light does not increase even if a large amount of current is applied to LED because a coefficient of thermal conductivity of printed substrate is small. However, if a metal substrate is employed, it can keep the surface temperature of LED at about 85 degrees C. because a coefficient of thermal conductivity of metal substrate is large. This means that a metal substrate effectively works as a heat sink and radiator plate, and the surface temperature of LED is decreased, and it is possible to increase driving current and simultaneously to expand the amount of light by the decrease in the surface temperature. As just described, in order to realize decrease in surface temperature of LED, the heat given off from LED has to be released outside as much as possible in some way.
Although a metal such as Al or Cu, a metal alloy and a ceramic such as aluminum oxide or AlN are nominated as a mounting substrate having superior radiation performance, Al which is lighter in weight attracts attention from ecological thinking. Al is a little inferior in coefficient of thermal conductivity compared to Cu, but it is the most important point that Al is cheaper and lighter in weight.
However, the metal substrate has a problem which relates to heat expansion coefficient. Some concrete numerical values are shown below.
Al: 23-25 ppm/degrees C.
Semiconductor element: 3.5 ppm/degrees C.
Chip resistor: 7.0 ppm/degrees C.
Chip condenser: 10.0 ppm/degrees C.
Solder: 23 ppm/degrees C.
By mounting LED on a metal substrate, it is possible to release a heat given off from LED, but it becomes overloaded with the weakest portion because of the difference in heat expansion coefficient between Al and LED as can be expected from the data.
FIG. 15, for example, shows a principle and a state that a chip element is mounted on a metal substrate. Arrows described in FIG. 15 indicate heat expansion coefficient, or expansion and contraction to temperature. Since an expansion of the metal substrate is larger, it burdens the solder, and thereby the acuter an angle between a side surface of the solder and a back side surface of the element is, the larger a load of portion illustrated by circle becomes.
In particular, LED package that LED is encapsulated in ceramic package or that LED mounted on lead frame is sealed with resin is mounted on the substrate through the solder in tentis of mounting technology, but the solder is burdened with large load due to large heat expansion coefficient in the Al substrate. In early times, the solder withstands the load. However, as thermal cycle number applied to the solder is growing with the clock ticking away, finally, a solder crack occurs and a circuit remains open, so that there is a possibility that LED becomes nonluminescent. Although the lead frame can absorb the stress applied to it by flexibility of the lead, in case of encapsulating LED in the ceramic substrate, because the structure that an electrode is deposited on the back surface of the ceramic substrate is generally employed, the solder disposed on the back surface of the electrode is burdened with load and the solder crack may be induced.